The former Health Secretary Alan Milburn revealed today that he had been questioned by detectives over the cash-for-honours affair.
Mr Milburn said he had not been interviewed under caution, and police had stressed he was being treated as a witness.
He said in a statement: "Following a request from the police, I have been interviewed as a witness. The police stressed I was not a suspect, and the interview did not take place under caution."
Mr Milburn was responsible for running the early stages of Labour's last election campaign, which was partly funded by loans at the centre of the controversy.
The revelation came after it emerged that Chancellor Gordon Brown and other Labour figures had been contacted by police and asked to provide written details of what they had know about the loans.
A Labour source said today: "The police haven't suggested interviewing (Deputy Prime Minister) John Prescott, Gordon Brown or other members of the NEC and Cabinet.
"They have simply asked those involved in the last election to declare formally in writing what they knew about the loans."
It was "well known" that Mr Prescott and Mr Brown "knew nothing" about the loans, according to the source.
"No-one has a problem if the police want to dot all the i's and cross all the t's," they added.
Detectives are believed to have made their request in a letter which was received by Mr Brown - who has publicly denied knowledge of the loans - over the past few days. He is not thought to have replied yet.
It is understood that Tony Blair has not received a letter from police asking for details of what he knew.
Asked today whether Mr Blair had been contacted by police over cash-for-honours, the Prime Minister's official spokesman said: " Nothing has changed."
Mr Blair has previously admitted having been aware of the loans, insisting he took responsibility for what had happened.
Mr Milburn resigned as Health Secretary, citing personal reasons, in 2003, but returned to the Cabinet in September the following year as Chancellor of the Duchy of Lancaster.
He was initially given responsibility for running Labour's 2005 election campaign, but it was perceived to have started poorly and Mr Brown took over. Mr Milburn, MP for Darlington, now sits on the backbenches.
Today's news - coming just weeks after former Tory leader Michael Howard announced that he had been quizzed - will again heighten speculation that the Prime Minister himself faces being interviewed by detectives.
Members of Mr Blair's inner circle, including director of Government relations Ruth Turner and, reportedly, chief of staff Jonathan Powell, have already been questioned.
The Scotland Yard probe was sparked by claims earlier this year that wealthy Labour backers were being rewarded with seats in the House of Lords in return for providing secret loans.
The scope of the inquiry was then widened to cover similar claims about the Conservatives. Three people - including Labour's unofficial fundraising chief, Lord Levy - have been arrested since April in connection with the probe, with many others questioned. However, so far there have been no charges.
Concern grew at the weekend that Lord Goldsmith, who was appointed by Tony Blair in 2001, could be called on to play a pivotal role in deciding whether his political colleagues face criminal charges.
The Attorney General's personal consent is required to proceed with prosecutions for certain types of offences, including corruption.
He is also usually consulted by the Crown Prosecution Service over high-profile and particularly complex cases.
Despite complaints from opposition parties, Lord Goldsmith has refused to rule out playing a role in any cash-for-honours decisions. However, yesterday he pledged to bring in independent counsel and suggested their advice could be published to demonstrate the process was not biased.Reuse content